<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            padding: 0;
            margin: 0;
        }
        
        .wrap {
            width: 300px;
            margin: 100px auto 0;
        }

        table {
            border-collapse: collapse;
            border-spacing: 0;
            border: 1px solid #c0c0c0;
            width: 300px;
        }

        th,
        td {
            border: 1px solid #d0d0d0;
            color: #404060;
            padding: 10px;
        }

        th {
            background-color: #09c;
            font: bold 16px "微软雅黑";
            color: #fff;
        }

        td {
            font: 14px "微软雅黑";
        }

        tbody tr {
            background-color: #f0f0f0;
        }

        tbody tr:hover {
            cursor: pointer;
            background-color: #fafafa;
        }
    </style>
</head>
<body>
    <div class="wrap">
        <table>
            <thead>
                <tr>
                    <th>
                        <input type="checkbox" id="j_cbAll" />
                    </th>
                    <th>商品</th>
                    <th>价钱</th>
                </tr>
            </thead>
            <tbody id="j_tb">
                <tr>
                    <td>
                        <input type="checkbox" />
                    </td>
                    <td>iPhone8(南京)</td>
                    <td>8000</td>
                </tr>
                <tr>
                    <td>
                        <input type="checkbox" />
                    </td>
                    <td>iPad Pro(北京)</td>
                    <td>5000</td>
                </tr>
                <tr>
                    <td>
                        <input type="checkbox" />
                    </td>
                    <td>iPad Air(东京)</td>
                    <td>2000</td>
                </tr>
                <tr>
                    <td>
                        <input type="checkbox" />
                    </td>
                    <td>Apple Watch(纽约)</td>
                    <td>2000</td>
                </tr>
            </tbody>
        </table>
    </div>
    <script>
         /* 
           需求: 
                 1. 全选，全不选。复选框，跟随全选状态变化。
                 2. 复选框点击的时候，复选框选中的个数和总数比较
        */
        // 全选按钮

        let j_cbAll = document.querySelector('#j_cbAll');
        let inputs = document.querySelector("#j_tb").querySelectorAll("input");


        //1.复选框，跟随全选状态变化。
        j_cbAll.onclick = function () {
            for (let i = 0; i < inputs.length; i++) {
                inputs[i].checked = this.checked;
                
            }
        }

        // 2.复选框点击的时候，复选框选中的个数和总数比较
        for (let i = 0; i < inputs.length; i++) {
            inputs[i].onclick = function () {
                // 判断复选框中被选中的个数
                let sum = 0;
                for (let j = 0; j < inputs.length; j++) {
                    if (inputs[j].checked) {
                        sum++;
                    }
                    
                }
                // 判断复选框和总数
                if (sum == inputs.length) {
                    j_cbAll.checked = true;
                }else{
                    j_cbAll.checked = false;                }
            }
            
        }
    </script>
</body>
</html>